Mutations (int-c) of bacteriophage lambda have been found to exhibit the following characteristics (a) high rate of int gene expression under repression; (b) low activity of xis gene under derepression; (c) some xis activity under repression. More precise mapping should show whether the int-c mutation and the origin of transcription actually lie within the xis gene. The frequency at which int-c prophages migrate to secondary attachment sites will be estimated from a study of double lysogens made by superinfecting int-c lysogens. Transcription of the biotin gene cluster of E. coli is regulated by two genes (birA, birB) that lie about 2 minutes apart, near ilv and argH, respectively. BirA mutants are defective in extractible holoenzyme synthetase activity. BirAts mutants will be screened for a temperature-sensitive holoenzyme synthetase and the role of this enzyme in repression will be examined in vitro.